In recent years, content distribution systems that use Digital Rights Management (DRM) techniques are becoming common. With DRM, encrypted content and a license are distributed to a user terminal. The license includes a content key and one or more content usage conditions, and the user terminal judges whether or not the content is able to be used in accordance with the usage conditions, and when the content is usable, decrypts the encrypted content using the content key and plays the content.
When a playable time period is listed as a usage condition, the judgment of whether the content is playable by the user terminal is made based on time information provided by a clock in the terminal. Consequently, in order to perform the judgment appropriately, it is necessary for the time information provided by the clock to be accurate.
However, it is possible that the clock may provide incorrect time information due to, for instance, power to the clock being temporarily cut. In such a case, the user terminal will be unable to make an appropriate judgment as to whether or not the content is usable, and will potentially permit usage of content when usage of the content is not actually permitted.
Patent Document 1 discloses a technique for solving this kind of problem. With the technique of Patent Document 1, usage of content is prohibited when a clock malfunction such as power to the clock being cut is detected and it is judged that the clock cannot provide accurate time information.    Patent Document 1: Japanese Patent Application Publication No. 2003-272286